1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to an apparatus and method for determining a fixed point in a mobile communication system that are capable of adaptively changing a fixed point according to an output range of a target device when a floating point is converted into a fixed point.
2. Description of the Background Art
In general, when a device is designed, a system designer must check the performance of the device and/or whether the device is operable by using simulation after applying an operation algorithm. If the performance of the device does not extend to the desired level, the operation algorithm should be corrected or a parameter adopted to the corresponding algorithm should be changed to find an optimum value that will provide the desired performance.
Simulation is executed by a tool or a program language. A primary object of the simulation lies in an algorithm comparison and a performance checking. In these stages, the simulation is performed without limiting the number of bits, which is called a floating point simulation.
However, in a stage where the device is actually implemented, the number of bits is limited because there is a limitation to the size of the hardware. But, the limitation to the number of bits leads to the degradation of device performance. That is, the number of bits and the performance have a tradeoff relation to each other.
Therefore, when the process moves from the simulation stage to the device implementation stage, a suitable fixed point is needed in consideration of the size of the hardware and the degree of performance degradation. The fixed point is determined through a fixed point simulation at the simulation stage. The fixed point determination involves determining the number of saturation bits and the number of truncation bits.
Once a suitable fixed point is determined through the above-discussed operation, a hardware is designed by using the determined fixed point at the device implementation stage, thereby completing the desired device.
In designing a system, the system designer first performs a floating point simulation for a digital data and then performs a fixed point simulation to convert the floating point into a fixed point. At this time, the system designer performs the simulation such that the number of bits and the position of the saturation and truncation bits are determined in an optimum state while changing the number of bits and the saturation point, thereby determining a fixed point.
The position of the fixed point, that is, the optimum number of bits and the position of the saturation and truncation bits, varies depending on an output range of a target device.
For example, an output of a despreader in a third generation partnership project (3GPP) downlink varies depending on a spreading factor (SF) and the number of orthogonal channels. Accordingly, in case of implementing a fixed point hardware for the output of the despreader, the number of bits and the position of the saturation bit and the truncation bit are changed.
In a limited fixed point hardware, many bits are required to cover all the output range of the despreader. In this respect, however, if the number of bits of the fixed point is continuously increased, the hardware size is increased and becomes complicated.
In addition, as mentioned above, the optimum number of bits and the position of saturation bit and truncation bit vary depending on the range of an output signal of the target device. Thus, the determination of the fixed point in this situation inevitably causes the degradation of performance of an overall system.